Lightweight mirrors of the “SPS-Alpha” solar power satellite concept focus sunlight for conversion into a microwave beam down to Earth.
Preparations for launch of a Falcon 9 rigged to flight test a nascent flyback capability in its first stage drew close attention from solar power satellite (SPS) advocates meeting here, who know that low-cost reusable launch is one key to realizing their dream of providing abundant electric energy from space.
While they are taking different approaches to developing SPS, the small but international group of participants at the SPS 2014 conference here agreed that their goal continues to be an end to the increasingly dangerous struggle to meet the energy needs of a growing world population. They see space solar power as an alternative to the environmental fallout from extracting and burning fossil fuel, and the military cost of securing supplies in unstable regions.
Like California-based SpaceX, the(JAXA) is conducting research into reusable launch as a way to cut the cost of space launch drastically. Japan is the only nation that has made beaming solar power collected in space back to Earth a goal of its space policy, and JAXA engineers calculate reusable launch is one way to reduce the up-front investment needed to put gigawatt-class power stations in geostationary orbit.
“We need a reusable launch system,” says Susumu Sasaki of Tokyo City University, a professor emeritus at JAXA who has studied the relationship between launch costs and the cost of power delivered from space.
Using a 2003 JAXA reference model with a 1-gigawatt station weighing 10,000 tons, Sasaki says power would cost a prohibitive $1.12/kwh at a launch cost to low Earth orbit (LEO) of $10,000 per kilogram. That is in the ballpark of what space launch costs today. Cut that to $1,000 a kilogram—in the ballpark for a reusable launch vehicle (RLV)—and electricity from space drops to 18 cents/kwh.
The SpaceX RLV work, which includes prototype landing legs on the current Falcon 9 taking cargo to the International Space Station (see photo on page 25) and using the rocket’s engines to control the first stage’s return to a splashdown in the Atlantic, is but one development in the fast-changing worldwide spaceflight endeavor that holds promise for space solar power.
Sasaki also cites the need for an orbital transfer vehicle (OTV) to move SPS hardware from LEO to the geostationary Earth orbit (GEO) where space power systems would operate, a development that meshes nicely with’s efforts to develop a high-power solar electric propulsion system for deep-space exploration (AW&ST March 31, p. 26).
Such a system would shuttle “like a Ferris wheel,” in Sasaki’s analogy between LEO and GEO, delivering 50 tons a year in the JAXA model with a four-month round trip. Overall, the JAXA approach—which already has a prototype robotic assembly device aggregating simulated power-converter units into larger structures on the ground at Tsukuba Space Center near Tokyo—would require 15 RLVs and more than 200 OTVs to build a power station in GEO, according to Sasaki.
John Mankins, a formerchief technologist who has worked with Kobe University professor Nobuyuki Kaya for decades on SPS, has devised a modular approach that would take a small SPS prototype into LEO, increase its capability and then upgrade it to megawatt-class stations in GEO. Parts of the “SPS-Alpha” concept (see illustration on page 24), outlined in great detail in a new book by Mankins entitled The Case for Space Solar Power, match up well with the modular “satlet” self-assembly concept the U.S. (Darpa) is pursuing to lower the cost of military satellites (AW&ST Jan. 20, p. 24).
To help fund development of the mass-produced components that would self-assemble in LEO and later GEO to increase capability, Mankins proposes a commercial approach that would sell electricity from the beginning. Users of the early systems in LEO could attach their payloads to the system in place of the power transmitters that are the ultimate goal, enabling much more powerful—hence capable—systems than exist today.
“You have costs, but you also begin to have revenues, because these systems are directly applicable to GEO communications satellites,” says Mankins, who co-chaired the SSP 2014 conference with Kaya. “They are directly applicable to all manner of LEO communications satellites, Earth-observing satellites and so on.”
Unlike Japan, which is working toward an SPS orbital test, and China, the U.S. has no government program supporting SPS development. But last year the Naval Research Laboratory (NRL) thermal vacuum ran a small program to test a lightweight prototype device melding a photovoltaic cell and a flat radiofrequency transmitter in a “sandwich” assembly that lends itself to the evolving space power station architectures and the mass production that would be needed to hold down the cost.
Paul Jaffe, the NRL engineer who put together the demonstration, says that aside from being the first test of SPS technology in space-like conditions, it also gives a data point for forecasting the economics of space-based power.
“It gives you kind of a rough estimate of what the cost is going to be, and it really just considers four factors,” Jaffe says. “We’ve talked a lot during this conference about how the cost of launch figures very prominently into whether SPS is likely to be economically feasible; the cost of the satellite [is important] as well [as the satellite service life]. This watts per unit kilogram is critical and probably the most difficult to quantify, which is one reason why the research we did with the module development is helpful in establishing this empirical basis.”
Last fall Ge Changchun, a Chinese academician who conducts SPS research at the University of Science and Technology in Beijing, told the International Astronautical Congress that China’s work in the area was underfunded because of the focus on human spaceflight. Since then, the government has paid more attention, he said here. Other attendees say the annual expenditure on the research in China has reached an estimated $30 million, which exceeds that of Japan.
Ge gave a detailed technical presentation on the Chinese SPS program, including his own focus on materials for the enormous but lightweight spacecraft that would be needed to collect solar energy in GEO. Although China is pursuing both laser and microwave power transmission options, there appears to be a growing consensus that microwaves in the 2.45 GHz or 5.8 GHz regions are the preferred wavelengths to pursue because of their all-weather capability, less-rigorous pointing requirements and other factors.
At those microwave wavelengths, conference participants agreed, there is not a safety risk in beaming huge amounts of power down from GEO-based power satellites. Birds could fly through the beams without injury and the huge rectennas set up to receive the microwaves and convert them into electricity would allow enough sunlight to pass through to the ground to support some kinds of agriculture in the proper climate zones.
But the preferred frequencies are already used for scientific research, and the International Telecommunications Union would need to allocate spectrum for SPS. Conference participants noted that the ITU has raised questions about SPS spectrum requirements that need to be addressed in time for the organization’s World Radiocommunication Conference in November.
There was also an appreciation that development of SPS should be incremental, both for technical reasons and to avoid “sticker shock” by those who hold the public and private purse strings. Additionally, other uses need to be found for the technology to broaden support for its development, as Mankins suggests.
Isabelle Dicaire, a physicist with the European Space Agency, outlined studies that show both microwaves and lasers from space could literally weaken dangerous hurricanes and other tropical cyclones by heating the water in them with microwave radiation to change the thermal dynamics or by using lasers to seed rainfall in a storm’s outer walls to weaken the strength of its rapidly rotating eye. Given the $100 billion cost of Hurricane Katrina in Louisiana, she said, the cost of a system that could mitigate cyclones and provide a space-based power source might be more acceptable.